1. Field of the Invention
The present invention relates to a laser projection apparatus for projecting an image onto a screen by two-dimensional scanning of light, the apparatus being applicable to image projection apparatuses such as pocket projectors, data projectors and rear projection televisions.
2. Description of the Related Art
Small-size image projection apparatuses for projecting an image by two-dimensional deflection scanning of light derived from a light source have been proposed in various types (e.g., the following Patent Documents 1 and 2). Generally, galvanomirrors or MEMS (Micro Electro Mechanical Systems) mirrors are used as an optical scanning means. In order that a galvanomirror or MEMS mirror is used with high speed and large amplitude (i.e., mechanical deflection angle), a drive method using resonance is employed. However, it is known that using such a resonant drive method would involve decreasing in the scanning velocity in the vicinities of maximum and minimum values of amplitude (i.e., peripheries of the image). This leads to problems, as commonly known, that the brightness of screen becomes higher in its peripheries than in its center, or that there may occur differences in resolution between the center and the peripheries on condition that the modulation rate of the light source is maintained constant.
For elimination of nonuniformities in brightness and resolution with a deflection scanning means using a resonant drive mirror, there is a need for applying a successful compensation by using an optical system to achieve a velocity uniformity on a scanning surface. More specifically, it is preferable that the projection optical system has an f-arcsine θ characteristic.
A projection optical system according to Patent Document 1, which is composed of two mirrors and one refractive lens, or three mirrors, can fulfill the compensation of the velocity uniformity or distortion.
A projection optical system according to Patent Document 2, which is composed of four mirrors, or an integrated-type prism having four reflecting surfaces, can fulfill the compensation of the velocity uniformity or distortion.
The related prior arts are listed as follows: Japanese Patent Unexamined Publications (koukai) JP-2006-178346, A and JP-2004-252012, A.
However, in the optical system of Patent Document 1 described above, in which a plurality of scanning mirrors are used as a reflecting optical system, there arises a need for distances between the scanning mirrors to separate rays of light from one another, and optical elements increase in size with increasing distances from the deflection scanning means so that the whole projection optical system inevitably increases in size.
Also, in the optical system of Patent Document 2 described above, a light source image which is once formed in an optical path of the projection optical system is projected onto a screen. Therefore, in spite of four reflecting surfaces used in the system, the final-stage reflecting surface is not so large in size, compared with the system of Patent Document 1, but there are some unavoidable problems, for example, 1) the optical system in itself is increased in size because of as many as four scanning mirrors, 2) it is difficult to position four curved-surface mirrors with high precision, and 3) an optical system up to the light source image is increased in size because the light source image to be once formed in the optical path is distant from the deflection scanning means.